1. Field of the Invention
The present invention relates to broadband radio frequency receiver systems and more specifically to narrowband interference suppression circuits for use with such receivers.
2. Description of the Prior Art
Broadband signalling, in which radio frequency energy to be transmitted is spread over a wide band of frequencies, is frequently employed in radar, sonar, navigation, communications, control, and identification systems to reduce: detectability of such transmissions by receivers other than those intended to receive the transmissions; intended receiver susceptibility to interference; or interference by such signals to other receivers. Such spread spectrum transmissions ordinarily operate at an advantage in the presence of narrowband noise. However sufficiently strong narrowband interference will comprise reception of the spread spectrum signals and may render the intended receivers inoperable. In order to overcome this potential problem, suppression means auxiliary to the basic receiver demodulation may be employed to suppress the narrowband interference when this inteference exceeds the level at which the receiver demodulator performance is seriously affected.
A number of approaches to this type of narrowband suppression have been suggested.
In one approach, a narrowband tracking loop, such as a phase-locked loop, is used to acquire a narrowband component within the broad input frequency band and subtract this component from the overall input signal. This system suffers from the fact that the broadband signal acts as a noise signal which disturbs the operation of the tracking loop. Furthermore, when more than one narrowband interference component is present, the loop subtraction maay operate unsatisfactorily. A multiple-loop cancellation system might be employed in such situations, but would be highly complex if not impractical. In addition this approach would be ineffective where sufficiently rapid phase or frequency modulation in the interfering signal is encountered.
Another closed-loop cancellation approach utilizes a filter which estimates the narrowband interference component in the received signal and subtracts this estimate from the input. The filter parameters are modified in an adaptive manner to minimize correlation of the narrowband components in the supression system output with corresponding components in the input. This approach requires complex circuitry to implement the filtering and adaptation functions.
Still another approach utilizes open-loop techniques wherein the input signal is measured and processed to reduce the interference component detected by the measurement. Such methods usually require a spectral analysis of the input signal which reveals the interference as peaks in the broadband measured spectrum. The peaks are attenuated or removed from the overall spectrum and an inverse spectral analysis operation is performed to transform the spectrum back into a time-domain waveform. This approach, which is referred to as a "frequency-domain excision" of narrowband interference, is described by R. M. Hayes and C. S. Hartmann in an article entitled "SAW Devices for Communications", appearing in the Proceedings of the IEEE for May, 1976, pp. 664-669 and L. B. Milstein and P. K. Das in an article entitled "Spread Spectrum Receiver Using Surface Acoustic Wave Technology", appearing in the IEEE Transactions on Communications for August 1977, pp. 841-847.
Frequency-domain excision systems employ various means for obtaining signals which represent the spectrum of the input waveform. In systems employing sampled-data, or discrete time, spectral analysis, the input signal is repetitively sampled and subjected to analysis by means of a Discrete Fourier Transform (DFT). If certain Fourier coefficients are large due to narrowband signal components of the input, they may be reduced or eliminated after which the remaining information is reconverted to a time-domain format. In a variation of this method, a series of N input signal samples is multiplied by a discrete waveform which has a quadratically varying phase-versus-time equivalent to a linear frequency modulation or "chirp". The product signal is then passed through a linear sampled-data filter having an impulse response with 2N-1 samples and a complex exponential form with a quadratic phase identical to the pre-filter multiplication signal except for its sign. An appropriately selected N-sample segment of the output of the filter is then multiplied by a delayed version of the previously mentioned discrete waveform to obtain a discrete signal comprising a sequence of the Fourier coefficients.
Such chirp transform spectral analysis employing analog processing means has been described, for instance, by R. M. Hayes et al in "Surface-Wave Transform Adaptable Processor System", in the 1975 Proceedings of the Institute of Electrical and Electronic Engineers Ultrasonics Symposium, pp. 363-370; by O. W. Otto in "The Chirp Transform Signal Processor" in the 1976 Proceedings of the Institute of Electrical and Electronic Engineers Ultrasonic Symposium, pp. 365-370; and in U.S. Pat. No. 4,049,958.
There are several difficulties with the chirp transform as used in the aforementioned systems. The filtering operation within the transform, for instance, is limited to a duty factor of less than 100%. Continuous or 100% duty factor transforms and their inverses are necessary to provide the fully reconstructed time-domain receiver input with suppressed interference. This duty factor problem may be overcome by using several switchable filters in a conventional chirp transform system, but this would entail considerable complexity in the required multiple closely matched filters and switching circuits. Alternatively, a single filter might be used with a 100% duty factor, but internal interference components which are thereby generated in the Fourier transformation and its inverse may themselves seriously interfere with proper receiver operation.
Systems applying 100% duty factor modified chirp transforms are described by T. F. Quatieri, Jr. in "Short-Time Spectral Analysis with the Conventional and Sliding CZT", Institute of Electrical and Electronic Engineers Transactions on Acoustics, Speech, and Signal Processing, Vol. ASSP-26, No. 6, December 1978. These systems perform signal reconstruction by interpolation and summation techniques. Fully reconstructed time domain signals with suppressed interference are not realized;
A more serious difficulty with frequency-domain excision employing conventional chirp transform techniques is the problem of coherent reconstruction of the time-domain waveform after excision. In both the discrete and analog chirp transforms, a segment of the input signal is gated to produce a finite duration signal representing the Fourier transform of the input segment. The inverse chirp transform operates in a similar fashion so as to produce a segment of the interference-suppressed output waveform. Thus when the receiver input is present continually, it must be segmented and processed through chirp transform excision circuits in blocks, and the sequential output segments thereby produced must be combined with the proper phase relationships and without significant transients. For continuous or "real time" excision using analog chirp transforms, this problem is difficult to solve in practice. Furthermore, the problem is compounded by the necessity for complex switched filter configurations needed to produce 100% duty factor operation. U.S. Pat. No. 4,287,475 discloses a system employing a conventional sampled-data chirp transform that accomplishes the desired reconstruction after excision. This system, however, due to the use of the conventional chirp transform, requires multiple switched chirp filters to realize the necessary 100% duty factor operation.
The present invention provides means for adaptively suppressing strong narrowband signals that might interfere with the reception of desired broadband signals on a continuous basis without apriori knowledge of the detailed characteristics of the narrowband interference.